This invention relates to a method for identifying characteristics of a contact lens, particularly the optical correction of the lens, and lenses useful in the method.
One type of contact lenses is commonly referred to as "spherical contact lenses", i.e., contact lenses designed to provide a spherical optical correction (or "power") to compensate for myopia (nearsightedness) or hypermetropia (farsightedness). Such contact lenses are also designed with fitting parameters, especially lens diameter and effective base curve. Accordingly, a prescription for a spherical contact lens will typically specify spherical correction (power), lens diameter and base curve. Using hydrogel lenses as an example, manufacturers typically market series of spherical hydrogel contact lenses, each series including lenses having common fitting parameters and offering powers in 0.25 or 0.50 diopter increments.
In addition to spherical lenses, there are contact lenses commonly referred to as "toric contact lenses", i.e., contact lenses having a toric optical zone that are designed to correct refractive abnormalities of the eye associated with astigmatism. The toric optical zone provides cylindrical correction to compensate for the astigmatism, with the cylindrical correction commonly referred to as "cylindrical power". The toric surface may be formed in either the posterior lens surface (back surface toric lens) or in the anterior lens surface (front surface toric lens). Whereas spherical contact lenses may freely rotate on the eye, toric contact lenses have some type of ballast to inhibit rotation of the lens on the eye is inhibited so that the cylindrical axis of the toric zone remains generally aligned with the axis of the astigmatism. For example, one or more sections of the lens periphery may be thicker (or thinner) than other sections to provide the ballast. Toric contact lenses are manufactured with a selected relationship (or offset) between the cylindrical axis of the toric optical zone and the orientation of the ballast. This relationship is expressed as the number of degrees (or rotational angle) that the cylindrical axis is offset from the orientation axis of the ballast; toric contact lens prescriptions specify this offset, with toric lenses generally being offered in 5 or 10 degree increments ranging from 0.degree. to 180.degree..
Since astigmatism requiring vision correction is usually associated with other refractive abnormalities, such as nearsightedness or farsightedness, toric contact lenses are generally prescribed, in addition to cylindrical power and axes offset, with a spherical correction and fitting parameters as for the aforementioned spherical contact lenses. Accordingly, a prescription for toric contact lens will typically specify spherical correction (power), lens diameter, base curve, cylindrical correction, and axes offset.
For the present invention, applicant recognized that it would be desirable to include on contact lenses identifiable markings that can be used to determine characteristics of the lens, particularly the optical correction of the lens. Preferably, the contact lenses are marked so that all relevant lens parameters, including fitting parameters of the lens in addition to optical correction, can be determined readily from the markings. For example, such markings can be used for in-line manufacturing processes, such as verification of lens parameters for product integrity purposes.
U.S. Pat. No. 4,976,533 discloses various prior methods for marking a toric contact lens, the markings being arranged in a manner that the ballast axis is identifiable. As an example, FIG. 2 of U.S. Pat. No. 4,976,533 illustrates a prior art toric contact lens including three visible line segments in a peripheral section of the lens, a first line segment lying on the ballast axis and two line segments arranged symmetrically about the first. Such lenses are useful for measuring the rotation of the lens on the eye, i.e., a practitioner can evaluate the lens when placed on the eye, and use the markings to measure any deviation of the lens from its intended rotational position, and to evaluate whether rotation of the lens on the eye is effectively inhibited as intended.
U.S. Pat. No. 4,194,814 discloses contact lenses that include identifying indicia engraved in a lens surface by subjecting the lens to a beam of laser radiation to form depressions in the lens surface. It is mentioned that the indicia may be machine-readable characters, and that the indicia may be used to identify manufacturer, material lot, and production run of a given lens, or to identify optical characteristics of lenses without actually measuring the lenses.